Method of dissolving plutonium dioxide in nitric acid using cerium ions



United States PatentO s 005 682 METHOD OF nrssoLvrN PLUTONIUM DIOXIDE 1N NITRIC ACID USING CERIUM IONS Archie S. Wilson, Richland, Wash., assignor to the United States of America asrepresented by the United States 5 1 Atomic Energy Commimion No Drawing. Filed Mar. 10, 1959, Ser. No. 798,554

6 Claims. (Cl. 23-145) This invention relates to a catalyst for the dissolution in nitric acid of plutonium dioxide, and to'the method 10 art. It is a dense, hard, refractory ceramic substance with great chemical inertnes's'which is advantageous in many respects since it resists corrosion as well as physical change in the environment of nuclear reactors. While there is no theoretical reason why it should not be used as the sole fuel in such reactors, for economic reasons it is usually mixed with uranium dioxide to make up the fuel elements which, in the solid fuel type v of reactors, are rods or slugs of the mixed oxides clad with a metal of low thermal neutron capture cross section such as aluminum or zirconium; in the liquid or so-called homogeneous type of reactors the mixed oxides are present in slurry form as is well known in the 8.11.

Like all fissionable fuels plutonium dioxide, in order to be useful, must be free within certain limits of impurities known as poisons, which are chemical elements that inter-fore with the neutronio reactionmf thc reactory fuel of greater purity which is often fuel from a pre-' vious run of the reactor which has undergone a refining process. Another reason why the refining of'plutcnium dioxide is required is safety, since some of the fission products are gases such as xenon which tend to rupture the metal protective cladding of solid fuel elements and thereby release dangerous radioactive substances into the reactor. Other good reasons exist for the desirability of refining plutonium dioxide but as these are 7 well known in the nuclear art a complete enumeration will not be attempted; the above-mentioned two con-- siderations are sutficient to show the utility of the invention which 'n addressedmonannimpmlementl inmthea.

presently known refining processes. I

A number of refining processes for plutonium dioxthe aqueous, or so-called wet chemical processes, in which plutonium dioxide is brought into aqueous solution and then refined by ionexchange, by liquid-liquid solvent extraction, or by some other wet chemical procedure. The most common, if not the only feasible method by which the plutonium dioxide is initially brought into such aqueous solution is by dissolving it in nitric acid, and it is to an improvement in this method that the present invention is particularly addressed.

Unlike uranium dioxide which dissolves readily in concentrated nitric acid, plutonium dioxide dissolves but slowly and incompletely; the only previously known for? oxiorzm" g agents f method for overcoming this inertness is to add 'as a catalyst fluoride ion in about 0.1 M concentration. The disadvantage of this procedure is that fluoride ion is highly corrosive and problems result from its use, either in removing it fron-r the solution which is difiicult, "or else the entire reactor and coolant system must be given an enhanced degree of corrosion resistance, for which changes in structural materials are usually necessary, often quite extensive in character. a

It has been suggested that the dissolution of plutonium dioxide in nitric acid could be improved by adding conventional oxidizing agents since the dissolution in nitric acid involves an oxidation of the plutonium from valence state IV to state VI as plutonyl nitrate (PuO (NO some of the nitric acid being reduced at the same time to a mixture of the oxides of nitrogen, the exact composition of which-hasnot been as yet fully established. This suggested expedient is objectionable since a burdensome removal problem would be thereby created far exceedingin magnitude the original problem of purificationsince the poisons" and other fission products are present in only very small concentrations compared to the stoichiometric amounts of oxidizing agents which would be necessary to bring about oxidation through their action.

It is the object, therefore, of the present invention to establish a method whereby plutonium dioxide can be brought into aqueous solution more quickly than by known methods without the useof the corrosive fluoride ion as a catalyst, or without the use of stoichiometric amounts of oxidizing agents.

establish a method whereby plutonium dioxide may be quickly dissolved in nitric acid without the use of fluoride ion as a catalyst.

It is a still more particular object of the invention' to establish a method whereby plutonium dioxide may be converted to plutonyl nitrate by reacting it with concentrated nitric acid without the use of fluoride ion as a catalyst, or without the use of stoichiometric amounts All the foregoing objects have been met by my discovery that plutonium dioxide may be dissolved in concentrated nitric acid and converted to plutonyl nitrate by the use of cerium ions as a catalyst. The temperature at which this is carried out is not critical but, of course, the known principles of thermodynamics as ap plied to reaction rates-hold good, and I have found the most practical temperature for the purpose to be at around 115C which is that of boiling concentrated aitric acids Ncr dees ieapparentlsmmakeaam md tler-rc ence what valence state the cerium is in when it is added as a catalyst. to the reaction mixture of plutonium dioxide are presently known, the chief group of which are ide' and concentrated nitric acid; both cerous and ceric nitrate give the desired catalytic efiect, as well as the double salt ammonium ceric nitrate, (NI-I Ce(NO Although ceric salts are well known conventional oxidizing agents, it is certain that the ceric nitrate does 0 not act in the usual way as a direct oxidizing agent as not a direct chemical attack. For this reason I refer to the amounts of cerium ions involved in this applicai 'it is moreparticuiarty an pbjecfiofrdre mventionno tion as "catalytic amoun as distinguished from stoichiometric amounts which may -be far greater by two or more orders of magnitude.

It has been suggested that if cerium is added in the cerous state it is oxidized by the nitric acid to the ceric state, whereupon the ceric ion acts to oxidize some of the plutonium, being itself reduced to the cerous state in the process, which is repeated over and over in a kind of cyclic scheme of successisve oxidations and reductions. 'While expla atio is pla si l I d n t. wish to be bound by it 01; any other theoretical explanation of my invention; the facts. are that if cerium is added in ny term r v n e state o he r io mixture d scri in. the a v sta ed c n en r on a greatly nh n d rate of d s lution of p t ni m dioxid n s. d the disso u on y e carri o c pleti n wi f r smaller quanti i o ni ri c d as the solvent than would be the case it no catalyst were employ d.

EXAMPLES 1 .TO 4

In Examples 1 to 3, 58 mg. of P110 and in Example 4 45 mg. of P110; were dissolved in '10 ml. of boiling on entra d nitri a id t about 115 wi h r l s shown in the following tabular form:-

From the foregoing it is apparent that the use of cerium in 'non-stoichiometric, or catalytic, amounts brings about a marked improvement in the dissolution of plutonium dioxide in nitric acid.

What is claimed is:

1. A method for bringing plutonium dioxide into aqueous solution comprising the addition of sufiicient cerium ions in catalytic amounts to the dissolving mixture of plutonium dioxide and nitric acid, to cause the mixture to be from 0.001 to 0.02 molar in cerium ions.

2. The method of claim 1 where the cerium is added to the mixture as ceric nitrate.

3. The method of claim 1 where. the cerium is added to the mixture as cerous nitrate.

4. The method of claim 1 where the cerium. is added to the mixture as ammonium ceric nitrate.

5. A method for dissolving plutonium dioxide in nitric acid c mp i in he addition f snificien ceri ions to the issol ing tuIe to cause the mix u e to e from 0.001 to 0.02 molar in cerium, ions.

6. A. method for conv rt P uton um dioxide n plutonyl nitrate comprising the. addition of sutficient ce iu i s t the reaction m ture of p uton m dioxide. an nitric acid to cause he ixtur o e rom 0.001 to 0.02 molar in cerium ions.

acid speeaed by cerium I [58 mg. P110; in 10 ml. conc. HNOfl Errol-0.005 M o E p. 2-0.015 M Ce Exp. 3no Ge Exp. 4 n0 Ce Iercent V 7 Percent Percent Percent Time, Hrs Dls- Time, Hrs. Dis: Time, Hrs. Dis- Time, Hrs. Dissolved solved solved solved i Only mg. ot PuO; was used.

References Cited in the file of this patent UNITED STATES PATENTS .70 5 Fermi M y 17, 5.5 2,785,951 Thompson et al. Mar. 17, 1957 2,833,617 Seaborg et al. May 6 19.58 2,873,169 Seaborg et al. Feb. 10, 1959 2,892,681 Crandall et a1. June 30, 1959 2,900,230 Larsen et a1. Aug. 18, 1959 OTHER REFERENCES Friend; Textbook of Inorganic Chemistry, vol.

VII, part III, page 302 (1926). Published by Charles Grifiin & (30., Ltd., London.

Technical Progress Reviews on Reactor Fuel Processing, vol. 1, No. 1, page 6, February 1958. Prepared by Argonne National Laboratory for U. S. Atomic Energy Commission.

Proceedings of the International Conference on the Peaceful Uses of Atomic Energy, vol. 9, pages 459-461,

Aug. 8-20, 1955, United Nations. 

1. A METHOD FOR BRINGING PLUTONIUM DIOXIDE INTO AQUEOUS SOLUTION COMPRISING THE ADDITION OF SUFFICIENT CERIUM IONS IN CATALYTIC AMOUNTS TO THE DISSOLVING MIXTURE OF PLUTONIUM DIOXIDE AND NITRIC ACID, TO CAUSE THE MIXTURE TO BE FROM 0.001 TO 0.02 MOLAR IN CERIUM IONS. 